The invention relates to a method and apparatus for monitoring and/or correcting of a physical characteristic or quantity ascertained from work pieces, for example, rotors for electrical motors, crankshafts, and the like produced in substantial numbers. The physical quantity may be modified during a or by a machining operation by determining the mean value of the characteristics of several finished work pieces. The invention is particularly directed to a method and apparatus for the monitoring of the state of unbalance of such work pieces.
In the operation of balancing machines, errors can be caused by many factors. Errors occur, for instance, in picking up the work pieces and placing them in processing machines, in the measurement of unbalance signals and in their transmission. Errors may also occur in the conversion of the unbalance signals and in particular, in compensating for an unbalance by removing or adding material. The individual errors may result in a residual unbalance of the work pieces leaving the balancing machine. Although this residual unbalance is in part a function of the initial unbalance of the original work piece, it is not proportional to the initial unbalance.
In the balancing with mass-centering machines of a large number of unfinished work pieces of the same kind, e.g., crankshaft blanks, methods are known for determining the unbalance before and after processing, as a typical physical characteristic of the work pieces in a fairly large number of individual work pieces. Customarily, these methods employ the component measuring method. All the unbalance values of these work pieces are combined in a mean value, or in the case of the component method, in mean values of two components. A comparison of these mean values of the unbalance obtained before and after the processing permits the determination of the unbalance changes caused by the processing. It is assumed here that, in the machining of work pieces which were approximately equal in the original state, the same amounts of material are removed at the same points.
In a known balancing method (German Pat. No. 975,210 and corresponding U.S. Pat. No. 2,804,775), centering marks are made on the work piece in a balance-centering machine in such a manner that not only the unbalance of the unfinished work piece, but also the expected average change in the unbalance due to the subsequent machining is taken into consideration.
However, forming the mean value and correcting the adjustment of balance-centering machines, made in dependence thereon, require a considerable effort. In particular, either no consideration is given, or consideration can be given only with extensive computing effort, to the fact that initially, only a relatively small number of finished work pieces is available for forming the mean value. This is primarily due to the fact that one attempts to obtain a mean value available for the correction as soon as possible, i.e., after having evaluated only a small number of produced work pieces. Because of the relatively small number of individual work pieces, the reliability of the mean value so formed is relatively low. It is not very satisfactory to continue working with such a mean value of low reliability when a larger number of finished work pieces is available.